Update deploy-preview.yml

Author: sserrata

.github/workflows/deploy-preview.yml

@@ -177,7 +177,7 @@ jobs:
         run: npx playwright install --with-deps chromium
 
       - name: Run visual diff
-        run: yarn ts-node scripts/sitemap-visual-diff.ts --preview-url ${{ needs.deploy.outputs.preview_url }} --summary-file visual_diffs/results.json --concurrency 4
+        run: yarn ts-node scripts/sitemap-visual-diff.ts --preview-url ${{ needs.deploy.outputs.preview_url }} --summary-file visual_diffs/results.json --concurrency 2
 
       - uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4
         if: always()